Optical device technology, e.g. electrochromic device technology, has advanced to the point where such devices can be fabricated using individual layers of materials that make up a “device stack” that is very thin, e.g., on the order of a few microns or even less than a micron in thickness. With such devices being very thin, uniformity of the substrate onto which they are fabricated is important, because the individual layers of the device stack may be on the order of tens or hundreds of nanometers thick. Non-uniformity in the substrate surface can translate into areas of the device stack that do not function properly or exhibit electrical shorting or other defectivity. Defectivity in the device stacks can often be correlated to the surface roughness of the substrate onto which the device stack is fabricated.
With small-scale production, e.g. in a research and development phase, a substrate can be carefully selected or fabricated for very high uniformity so that the above-described issues do not arise. As electrochromic device fabrication moves to large-scale production, it is more practical to purchase large area substrates, e.g. float glass coated with a transparent conducting film, from commercial sources that produce such substrates in large volume. The problem that often arises in such substrates is surface roughness of the pre-formed coatings thereon. For example, float (soda lime) glass is commercially offered with a bilayer coating on one surface. The bilayer includes a first layer which is a sodium diffusion barrier to prevent sodium ions from the glass from passing through it, and a second layer on the first layer which is a transparent conducting layer, e.g., a doped tin oxide such as indium tin oxide, fluorinated tin oxide, or similar transparent conducting coating. These coated glasses are well suited for a number of applications, including production of optical coatings thereon; however, for functioning device stacks on the order of a few microns or less in thickness, the surface roughness of these pre-formed coatings may be problematic at least for the reasons articulated above.
Also, there is an inverse relation between the desired properties of a glass substrate for optical coatings and some of the actual properties of the glass substrate when one moves to large scale production. For example, it is desirable to have a transparent conducting coating (e.g., film) with low sheet resistance across a glass substrate with an electrochromic device fabricated thereon. The lower the sheet resistance, the faster the electrochromic device may be able to switch. However, when moving to large scale substrate production, in order to produce highly reliable coatings, transparent conductors are generally made with larger grain size. Oftentimes these transparent coatings will have a low sheet resistance, but will also have a higher haze (light scattering) due to the larger grain size of the coatings. This haze is not a desired property of some final optical device products, e.g., electrochromic windows having electrochromic devices. For such windows, clarity and high contrast are important qualities. The larger grain size also contributes to the surface roughness, which is undesirable for the reasons described above. For context, transparent conductor coatings in large scale production substrates may have a surface roughness (Ra) of about 7 nm to 10 nm, and sometimes higher than 10 nm. For conventional applications, these surface roughness and haze properties may be considered well within acceptable levels or desired levels. For example, some optical device applications, such as photovoltaic cells, benefit from higher haze and roughness levels due to the increased scattering of incident light improving absorption efficiency. There may not be a need to polish a transparent conducting layer for these conventional applications.
There have been studies on polishing transparent conducting layers on glass. For example, indium tin oxide (ITO) layers have been polished using magnetorheological finishing (MRF) which resulted in a surface with surface roughness down to a few nanometers (see e.g., www.optics.rochester.edu/workgroups/cml/opt307/spr07/chunlin/, last visited Sep. 30, 2011). Also, the surface roughness of ITO layers has been reduced using KrF excimer lasers (see J. Vac. Sci. Technol. A 23, 1305 (2005). However, these techniques are highly specialized and prohibitively expensive to implement on large area substrates and/or in a mass production setting. One reported method of reducing haze in tin oxide coatings describes, rather than polishing the roughness down, filling in the valleys to smooth the overall contour of the coating (see U.S. Pat. No. 6,268,059). Compositions and methods that use acidic zirconia or colloidal silica for chemical mechanical polishing of ITO have also been reported (see US 2007/0190789). Despite these advances, there is a continuing need for new and improved methods of reducing haze and smoothing the surface of transparent conductive coatings.